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[Pd(NHC)(PEPPSI)]
(4 mol%)
R''
X
R''
N
+
H
N
R
R'
R
Cs
2
CO
3
DME, 80°C, 24h
or
Toluene, 110°C, 2 4h
R'
X=Cl,Br
R
R
R
R
Secondary alkyl amines:
NN
R
R
O
R
R
O
Cl
Pd
Cl
N
N
N
N
N
N
S
O
MeO
Cl
15:0%
4: 90%
15:36%
4:83%
15:28%
4:92%
[Pd(NHC)(PEPPSI)]
R=Me,NHC=IPr,15
R=Et,NHC =IPent,4
S
N
N
Et
N
N
Ph
15:0%
4:75%
15:23%
4:83%
Anilin es:
N
N
N
N
F
MeO
F
Me O
OMe
COMe
NC
CN
F
15:0%
4:96%
15:0%
4:92%
15:0%
4:67%
1 5:0%
4 :60%
Scheme 4.60
[Pd(IPent)(PEPPSI)] versus [Pd(IPr)(PEPPSI)] in amination.
(Scheme 4.60). A year later, in a very similar study,
170
Organ and co-workers
investigated the reactivity of [Pd(NHC)(PEPPSI)] complexes in the coupling
of anilines. Rate studies showed that anilines bearing electron-
withdrawing groups led to slower reactions. The authors reasoned that
this was inconsistent with the deprotonation of the amine being the rate-
limiting step, as was the case for the secondary alkylamines. Considering
that electron-donating groups on the aniline increased the rate and that
electron-withdrawing groups on the aryl chloride also increased the rate,
they concluded that the reductive elimination was most probably the lim-
iting step. In terms of reactivity, IPent was found to be more reactive than
IPr to couple anilines, as was the case for secondary alkylamines. More
interestingly, IPent was able to catalyse some reactions that were not pos-
sible using IPr (Scheme 4.60).
Recently, Nolan's group has been heavily involved in the development of
new well-defined Pd-NHC precatalysts to promote Buchwald-Hartwig cross-
coupling. In 2012, they investigated the use of the highly bulky yet flexible
IPr* ligand
171
in amination. Three new complexes were prepared:
[Pd(IPr*)(cin)Cl] (9),
172
[Pd(IPr*)(acac)Cl] (85)
173
and [Pd(IPr*)(3-Cl-pyr-
idinyl)Cl
2
](86)
56
bearing three different classical throwaway ligands de-
veloped by the groups of Nolan and Organ (Figure 4.37). [Pd(IPr*)(cin)Cl] and
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